1. Field of the Invention
The present invention relates to an image forming apparatus of the type transferring a toner image from a photoconductive element to a recording medium by way of an intermediate transfer belt. More particularly, the present invention is concerned with a full-color image forming apparatus capable of forming attractive images free from defects including local or vermicular omission.
2. Discussion of the Background
It is a common practice with a full-color image forming apparatus to sequentially transfer toner images of different colors from a photoconductive element to an intermediate transfer belt one above the other, and then transfer the resulting composite or full-color image from the belt to a paper or similar recording medium, as taught in, e.g., Japanese Patent Laid-Open Publication No. 2-282491 or 5-210316. In this type of apparatus, a secondary transfer roller is held in contact with the inner periphery of the belt. A paper transfer roller faces the secondary transfer roller with the intermediary of the belt. The belt carries a toner image thereon. While a bias for image transfer is applied to the sheet transfer roller, the transfer roller is pressed against the portion of the belt contacting the secondary transfer roller. As a result, the toner image is transferred from the belt to a paper or similar recording medium being passed between the belt and the roller. The image transfer from the belt to the paper is referred to as secondary transfer, as distinguished from primary transfer from the photoconductive element to the belt.
However, the conventional apparatus having the above construction has the following problems. When the paper transfer roller is pressed against the intermediate transfer belt for the secondary transfer, the transfer roller locally presses the toner image deposited on the belt in the form of a layer and thereby compresses it. As a result, the parting ability of the toner and the surface of the belt from each other is lowered. This causes the toner to locally remain on the belt after the secondary transfer. Therefore, the image transferred to the paper is partly lost in a vermicular configuration.
Further, an air gap exists around the inlet of the nip between the secondary transfer roller and the paper transfer roller. Electric lines of force (electric field) are formed in the air gap due to a transfer bias. Consequently, the toner is scattered around at the position upstream of the nip and caused to deposit on the paper, resulting in a blurred or otherwise defective image.